1. Field of the Invention
The present invention relates to a solid-state imaging apparatus.
2. Description of the Related Art
In a solid-state imaging apparatus, such as a CMOS sensor, in which pixels provided with a photoelectric conversion element are arrayed one-dimensionally or two-dimensionally and voltage outputs from the pixels are read by circuits provided on a column-by-column basis, an S/N ratio can be improved by amplifying voltage using the column-by-column read circuits.
It is important in terms of the S/N ratio that an amplifier (hereinafter referred to as a column amplifier) provided for each read circuit to amplify voltage will perform the amplification without compromising dynamic ranges of output signals from the photoelectric conversion elements. Also, in imaging of a very low-luminance object, the S/N ratio can be improved by amplifying signals using the column amplifier.
In an image sensing system, generally an amplifying ratio of the column amplifier is varied according to illuminance of the object. A column amplifier circuit with a variable amplifying ratio is disclosed in Japanese Patent Application Laid-Open No. 2005-175517.
However, with the technique disclosed in Japanese Patent Application Laid-Open No. 2005-175517, parasitic capacitance becomes non-negligible with decreases in pixel pitch, resulting in an error: a desired amplifying ratio is not available especially at high gain settings. The solid-state imaging apparatus has the following three problems.
(1) To compensate for decrease in the amplifying ratio caused by the parasitic capacitance, size of an input capacitor needs to be increased, resulting in an increased chip area.
(2) Capacitance between wires and capacitance between switch terminals, which can cause parasitic capacitance, are affected by manufacturing variations such as variations in film thickness of insulating films and film thickness of metal wiring layers, and thus the amplifying ratio is also subject to the manufacturing variations.
(3) Disturbance is liable to get introduced via parasitic capacitance and is noticeable especially at high amplifying ratios.